N-vinyl-2-pyrrolidone (NVP) is a monomer used for making crosslinked or uncrosslinked polyvinylpyrrolidones, vinyl pyrrolidone-vinyl ester copolymers, and other valuable polymers. The polymers are used in beverage clarification, hair care, pharmaceutical tablet binding, and other industrial applications.
NVP is commonly manufactured by reacting 2-pyrrolidone with acetylene, but non-acetylenic routes to NVP are available. In one approach, NVP is made by dehydrating N-(2-hydroxyethyl)-2-pyrrolidone (HEP), which is easily obtained by reacting gamma-butyrolactone with 2-aminoethanol.
The dehydrations are normally performed by passing a vaporized mixture of HEP and a diluent gas, usually nitrogen, through a heated reactor tube containing a catalyst. NVP is collected along with water, unreacted HEP, and by-products. The by-products usually include 2-pyrrolidone (“2-Py,” a hydrolysis product), N-ethyl-2-pyrrolidone (NEP), and nonvolatile materials (“heavies”). NEP is particularly undesirable because unlike 2-Py, it is not easily converted back to NVP. Moreover, pharmaceutical-grade NVP must be substantially free of NEP, and because NEP and NVP have similar boiling points, an expensive crystallization step is usually needed to separate them. NEP is seldom mentioned in the literature. However, there is at least one report of NEP being observed in an HEP dehydration process catalyzed by alumina (see U.S. Pat. No. 3,821,245 at column 4, line 17).
As noted above, an inert gas such as nitrogen or argon is commonly used to dilute the HEP in the dehydration process. U.S. Pat. No. 5,410,170 teaches (column 2, lines 6–10) that “inert gases, such as hydrogen, nitrogen, or steam, or the addition of inert organic solvents, or the use of reduced pressure can be advantageous.” The reference does not say what advantage is expected from using the inert gases, and Table 1 (columns 3–4) shows no examples in which the HEP is combined with nitrogen, steam, or any other diluent.
Despite the availability of a well-known non-acetylenic route to NVP from HEP, the industry would benefit from a process that delivers and sustains higher HEP conversion, better NVP selectivity, reduced NEP formation, and a lower level of non-volatile by-products. An ideal process would be easy to perform using conventional equipment and would use cheap, readily available starting materials.